1. Field of the Invention
The present invention relates to techniques for mounting optical frequency reference cavities in such a manner as to reduce the effects of mechanical vibration.
2. Problems in the Art
A common method of achieving optical frequency stability in a laser is to use a laser whose output frequency can be tuned with a control signal, and to stabilize (servo-lock) the laser frequency to a mode (or resonance) of a passive cavity, called an optical frequency reference cavity. Compared to the laser cavity, the reference cavity can be made far more stable, since it has no gain medium. The reference cavity might be a Fabry-Perot cavity such as a high finesse two-mirror cavity with a spacer between the mirrors formed of low thermal expansion coefficient material.
A number of techniques have been used to make the reference cavity even more stable. For example, the reference cavity mounting might include vibration absorbing damping elements, acoustic vibration isolation, temperature stabilized housing, etc.
Methods such as mechanical isolation (on a heroic scale), low pass filtering, or active anti-vibration approaches are sufficiently productive such that, by now several groups have developed visible optical sources with ˜Hz linewidths. Further progress has been very challenging—all the margins have been used up. In addition, the most successful approach, active anti-vibration techniques, is expensive and complicated to implement.
One of the present inventors conceived of an idea to mount a reference cavity vertically at a single central plane, rather than horizontally as is conventional. See J. L. Hall, “Frequency Stabilized Lasers: a parochial review,” Proceedings of the SPIE, 1837, 2-15 (1993). A second of the present inventors built and experimented with such a system. See: “Cryogenic system for a sapphire Fabry-Perot optical frequency standard,” Cryogenics 1996, Volume 36, Number 1, pp 13-16. However, the stability challenges that were caused by the single plane mounting were thought to be too large to overcome at that time. In addition, the mounting mechanism used a clamping collar, which, as it turned out, squeezed and distorted the spacer cylinder and degraded performance. Until the present invention was made, no one in the field of optical frequency reference cavities felt that the vertical orientation would be workable.
Thus, there exists a need for an alternative technique for stabilizing optical reference cavities, by mounting the cavities vertically at their horizontal geometrical midpoint.